1. Field of the Invention
This invention relates to carbide powders. In particular, this invention relates to a method for synthesizing micron- and submicron-sized, high purity, high surface area, cermet carbide powders from oxides using a high energy milling step.
2. Brief Description of the Related Art
Micron-, submicron-, and nanosized carbide powders are important materials for engineering applications. As used herein, xe2x80x9cmicron-sizedxe2x80x9d powders refers to powder particles wherein the mean particle size is equal to or greater than about 1.0 microns. xe2x80x9cSubmicron-sizedxe2x80x9d powders refers to powder particles wherein the mean particle size is in the range from about 0.1 to about 1.0 microns. xe2x80x9cNanosizedxe2x80x9d powders refers to powder particles wherein the mean particle size is less than about 0.1 micron (about 100 nm). In certain instances, micron- and and submicron-sized particles comprise many nanosized crystalline grains rather than a single, large grain. In these cases, the particles are referred to as xe2x80x9cmicron- and/or submicron-sized particles with nanostructuresxe2x80x9d or xe2x80x9cmicron- and/or submicron-sized, nanostructured particles.xe2x80x9d Nanosized and micron- and/or submicron-sized, nanostructured particles have a high fraction of atoms located at the grain boundaries of the particle. Such materials accordingly have different, and often advantageous properties compared to conventional particles having the same chemical compositions.
Currently, the primary process for the production of micron- and submicron-sized carbide powders is high-temperature, carbothermic reduction of the corresponding oxides by carbon powders. For example, most industrial silicon carbide (SiC) powders are manufactured via the Acheson process through carbothermic reduction of silicon dioxide (SiO2) by carbon powder at temperatures up to 2000 to 2300xc2x0 C. for 30 hours. A carbothermic method based on reduction of one or more metal oxides reacted with a binder material and a source of carbon is disclosed in U.S. Pat. No. 4,784,839 to Bachelard et al. Similarly, as described in UK Patent No. 811,906 (issued in 1959), industrial titanium carbide (TiC) powders are produced through reduction of titanium dioxide (TiO2) by carbon at temperatures ranging from 1700 to 2100xc2x0 C. for 10 to 20 hours.
Advantages of using high-temperature carbothermic reduction for the production of carbides include low cost of the oxide raw materials, and ease in scale-up for tonnage-level production. However, the final products have a wide range of particle sizes, and moreover are normally larger than one micron, due to high reaction temperatures and long reaction times. Milling after carbothermic reduction is required. Undesirable inhomogeneities are also frequently found in the stationary reaction mix. These inhomogeneities are due to diffusion gradients established during the reduction reaction, and require extensive milling and purification procedures in order to convert the as-synthesized products into high quality, sinterable powders. The SiC powder produced by the Acheson process, for example, has a large grain size and is contaminated with oxygen. Accordingly, there remains a need in the art for methods whereby homogenous carbide powders may be produced having a controlled and uniform size, without extensive milling and purification procedures.
Cermet materials area of particular industrial interest. Tungsten cobalt carbide (WCxe2x80x94Co), for example, is a well known carbide cermet, having been widely used in aluminum, titanium, steel, aerospace, automobile, electronics, oil and wood industries, and in military applications as metal-cutting tools, wire drawing dies, cold and hot rolls, punches and dies for blanking and extrusion, bearing, micro-twist drills for printed circuit boards and printer heads, complex components for aero engine fuel systems, rock-drilling bits, oil well drills, large valves for controlling sludge in the oil industry, slip gauges, compacting tools, measuring devices, fan blade and mixer blade cladding, hardfacings for a variety of wear components, tire studs, armor-piercing projectiles, and other applications.
Currently, nanostructured WCxe2x80x94Co cermet powder is commercially manufactured by the above-described conventional industrial process, or by spray conversion processing. Spray conversion processing consists of (i) preparation and mixing of aqueous solutions of the precursor compound (CoCl2 as cobalt source and H2WO4 and (NH3)WO4 as tungsten source); (ii) spray drying the aqueous solution to form a chemically homogeneous precursor powder, and (iii) thermochemical conversion of the precursor powder to the nanostructured end-product powder. The thermochemical conversion is conducted in a fluidized bed reactor at 700 to 900xc2x0 C. in a Co/CO2 atmosphere. The WCxe2x80x94Co powder produced is in the form of hollow, porous spheres 10-40 microns in diameter, comprising internal nanosized grains less than about 50 nm in diameter. There accordingly remains a need in the art for economical method for the manufacture of high quality, high surface area cermet materials such as tungsten cobalt carbide.
The above-described drawbacks and deficiencies of the prior art are alleviated by a method for the manufacture of high purity, high surface area, carbide cermet powders, comprising high energy ball milling a mixture of precursor powders, followed by annealing the milled powder mixture. The precursor powders are selected from materials suitable for the formation of cermets, for example silicon, titanium, thorium, hafnium, vanadium, chromium, tungsten, tantalum, niobium, and zirconium-containing materials. The precursors further include a source of carbon. Tungsten cobalt carbide powders produced by this method are submicron-sized (0.2 to 0.4 microns) with internal nanograins (10 to 40 nanometers in diameter). The synthesized powders may also include other nanophases such as TiC, VC, TaC, NbC, and the like. These nanostructured WCxe2x80x94Co powders produced in accordance with this method have furthermore been shown to produce components with much better wear resistance than those made of conventional WCxe2x80x94Co powders.
Preferably, carbon monoxide is removed from the reaction chamber during heating in order to drive the reaction to completion at low temperatures and/or short times. The high energy milling step serves to mix the oxide and carbon on a nanosized scale and to increase the reactivity of the reactants by increasing surface area, introducing structural defects and internal strains, and transforming the crystalline materials to an amorphous state. The method produces high purity, high surface area, micron- or submicron-sized carbide powders having a narrow particle size distribution and internal nanostructure. The method is conducted at low temperatures, for short processing times, and thus significantly lowers cost.